Chapter 1BoylestadElectronic Devices and Circuit TheorySome of the slides are modified for the consumption of this classSources used are : 1. Electronic Devices by FLOYD 2, Electronic Devices and Circuit Theory by Boylestad

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ION:An atom or molecule with a net electric charge due to the loss or gain of one or more electrons.Characteristics :Characteristics can be define by its physical attributes.*

DiodesThe diode is a 2-terminal device.A diode ideally conducts in only one direction.*AnodeCathode

Characteristics of a Diode: The characteristics of an ideal diode are those of a switch that can conduct current in only one direction.The ideal diode is a two-terminal device having the symbol and characteristics as shown in the Fig*

Semiconductor MaterialsMaterials commonly used in the development of semiconductor devices:Silicon (Si)/ Doped SiliconGermanium (Ge)/ Doped GermaniumGallium Arsenide (GaAs)/Doped GaAs*

A forward-biased diode showing the flow of majority carriers and the voltage due to the barrier potential across the depletion region.Forward-biased diode connections *

Effects of Forward/Revise Biasing on the Depletion Regiondepletion region.(No Bias)depletion region.(Forward Bias)depletion region.(Reverse Bias)*

Effects of Forward/Revise Biasing on flow of Majority CarriersThe extremely small reverse currentin a reverse-biased diode is due tothe minority carriers from thermallygenerated electron-hole pairs.Forward BiasA forward-biased diode showing theflow of majority carriers and thevoltage due to the barrier potentialacross the depletion region.Reverse Bias*

Diode Operating ConditionsForward BiasThe electrons and holes have sufficient energy to cross the p-n junction.The forward voltage causes the depletion region to narrow.The electrons and holes are pushed toward the p-n junction.*

Diffusion currentDiffusion current occurs even though there isn't an electric field applied to the semiconductor.Drift currentDrift current depends on the electric field applied on the p-n junction diode.*

VOLTAGE-CURRENT CHARACTERISTIC OF A DIODEV-I Characteristic for Forward BiasForward-bias voltage is applied across a diode

Forward current is established

External resistor is added to limit the forward current to protect junction from overheating.

Zero volts across the diode, no forward current

Gradually increase in VBIAS will increase VFforward-bias voltage and forward current*

In the absence of an applied bias voltage, the net flow of charge in any one direction due to electron or holes in a semiconductor diode is zero.No Bias CurrentDue to movement of minority carrier the current that exists under reverse-bias conditions is called the reverse saturationcurrent and is represented by Is. Reverse Bias CurrentForward Bias CurrentAll current due to electron no reverse currentCurrent Established by Majority and Minority Carrier*

Up to now we have learned:

A diode is a pn-junction device.How the pn-junction physically behave.How does the current flow at the pn-junction._________________________________________________________

Next: How a diode can be modeled for circuit analysis (three different models)

Bias connections Diode approximations Ideal diode model Practical diode model Complete diode modelpn-junction Diode*

pn-junction DiodeLogic/Schematic Symbol*

Forward-Biasing CircuitThe positive terminal of the source is connected to the anode through a current-limiting resistor.

The negative terminal of the source is connected to the cathode.

The forward current (IF) is from anode to cathode as indicated.

The forward voltage drop (VF) due to the barrier potential is from positive at the anode to negative at the cathode.cathodeanode*

The negative terminal of the source is connected to the anode through a current-limiting resistor.

The positive terminal of the source is connected to the cathode.

The forward current (IF) is from anode to cathode as indicated.

The forward voltage drop (VF) due to the barrier potential is from positive at the anode to negative at the cathode.Reverse-Biasing-Circuitanodecathode*

Forward Biased Diode The diode behaves like a ON switch in this mode

Resistance R and diodes body resistance limits the current through the diode

VBIAS has to overcome VBARRIER in order for the diode to conduct

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Reverse Biased DiodeThe diode behaves like a OFF switch in this mode

If we continue to increase reverse voltage VB breakdown voltage of the diode is reached

Once breakdown voltage is reached diode conducts heavily causing its destruction*

Thermal Breakdown

Diode breakdown is caused by thermally generated electrons in the depletion region

When the reverse voltage across diode reaches breakdown voltage these electrons will get sufficient energy to collide and dislodge other electrons

The number of high energy electrons increases in geometric progression leading to an avalanche effect causing heavy current and ultimately destruction of diode*

Diode Approximations2. The Practical Diode Model1. The Ideal Diode Model3. The Complete Diode Model*

The Ideal Diode ModelThe ideal model of a diode:

Suppose Approximation is used and diode is considered as and represented by a simple switch (or wire).

When the diode is forward-biased, it ideally acts like a closed (on) switch (or wire/path is connected)

When the diode is reverse-biased, it ideally acts like a opened (off) switch (or wire/path is disconnected).

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The Ideal Diode ModelThe ideal model of a diode:

Suppose VF = 0V and IR = 0 then

For Reverse Bias Ideal Diode

VR = VBIAS *

The Practical diode ModelThe practical model of a diode:

Barrier potential caused by the depletion region is taken into account in the practical model

that is 0.7v for Si 0.3v for Ge For the current to flow the source voltage must need to over come the barrier.*

The Practical diode ModelThe practical model of a diode:Forward current can be determined by applying Kirchhoffs voltage law*

2. The Practical Diode ModelThe diode is assumed to have zero reverse current,The practical model is useful for:

Troubleshooting in lower-voltage circuits.Excepting the 0.7 V drop across a good diode

The practical model is also useful when basic diode used in the design circuits.*

2. The Practical Diode ModelForward current can be determined by applying Kirchhoffs voltage lawThe diode is assumed to have zero reverse current,The practical model is useful for:

Troubleshooting in lower-voltage circuits.Excepting the 0.7 V drop across a good diode

The practical model is also useful when basic diode used in the design circuits.*

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The Complete Diode ModelTin complete model approximation includes the followings:

The barrier potentialThe small forward dynamic resistanceThe large internal reverse resistance

The reverse resistance is taken into account because it provides a path for the reverse current, which is included in this diode model.

When the diode is reverse-biased, it acts as an open switch in parallel with the large internal reverse resistance as shown *

The Complete Diode ModelThe curve slopes because the voltage drop due to dynamic resistance increases as the current increases. For the complete model of a silicon diode, the following formulas apply:*

ExampleUse the practical model to determine the current in the circuit:3.4 mA*

Example:*

Example:*

Example:*

*Circuit analysisis the process of finding the voltages across, and the currents through, every component in the circuit.Diode circuit voltage measurements: (a) Forward biased. (b) Reverse biased.

Actual Diode CharacteristicsNote the regions for no bias, reverse bias, and forward bias conditions.Carefully note the scale for each of these conditions occurs.*

Zener RegionAt some point the reverse bias voltage is so large the diode breaks down and the reverse current increases dramatically.The voltage that causes a diode to enter the zener region of operation is called the zener voltage (VZ).The Zener region is in the diodes reverse-bias region.The maximum reverse voltage that wont take a diode into the zener region is called the peak inverse voltage or peak reverse voltage.*

Forward Bias VoltageThe point at which the diode changes from no-bias condition to forward-bias condition occurs when the electrons and holes are given sufficient energy to cross the p-n junction. This energy comes from the external voltage applied across the diode.The forward bias voltage required for a: gallium arsenide diode 1.2 Vsilicon diode 0.7 Vgermanium diode 0.3 V*

Temperature EffectsIt reduces the required forward bias voltage for forward-bias conduction.It increases the amount of reverse current in the reverse-bias condition.It increases maximum reverse bias avalanche voltage.As temperature increases it adds energy to the diode. Germanium diodes are more sensitive to temperature variations than silicon or gallium arsenide diodes.*

Temperature EffectsAs temperature increases it adds energy to the electronsGermanium diodes are more sensitive to temperature variations than silicon or gallium arsenide diodes.The reverse saturation current Is will just about double in magnitude forevery 10C increase in temperature.*

Resistance LevelsDC (static) resistance AC (dynamic) resistance Average AC resistanceSemiconductors react differently to DC and AC currents. There are three types of resistance:*

DC (Static) ResistanceFor a specific applied DC voltage (VD) the diode has a specific current (ID) and a specific resistance (RD).*

Example:*

Small-Signal Model of a Diode iD as a function of vD is non-linearTherefore tools of linear circuit analysis cannot be applied, in general, to circuits containing diodesLinear circuit analysis can be used to predict the change in current for a given change in voltage, provided the change is not very large. Such an approach is called a small-signal analysis. Diode bias voltage with small AC signal can be written aswhere VD and ID are dc bias values and vd and id are small-signal changes about the bias values

ORidrd = vd / idSmall-Signal Model of a Diode

AC (Dynamic) ResistanceThe resistance depends on the amount of current (ID) in the diode.The voltage across the diode is fairly constant (26 mV for 25C).Body resistance rB introduced by the connection between the semiconductor material and the external metallic conductor (called contact resistance)rB ranges from a typical 0.1 for high power devices to 2 for low power, general purpose diodes. In some cases rB can be ignored.In the forward bias region:In the reverse bias region:The resistance is effectively infinite. The diode acts like an open.*

Average AC ResistanceAC resistance can be calculated using the current and voltage values for two points on the diode characteristic curve.*

Effects of Operating Point on the Dynamic Resistance*

Effects of Operating Point on the AC Signal*

Defining the dynamic or ac resistanceThe designation Q-point is derived from the word quiescent, which means still or unvarying.

QuiescentORQ-pointQ-point Small AC signal applied at Q-Point*

resistance at each current levelExample:(c) Compare Static resistance VS. Dynamic resistance*

Diode Equivalent Circuit*

Reverse Recovery Time (trr)Reverse recovery time is the time required for a diode to stop conducting when switched from forward bias to reverse bias.*

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Diode Symbol and PackagingThe anode is abbreviated A The cathode is abbreviated K *

Diode TestingDiode checkerOhmmeterCurve tracerDiodes are commonly tested using one of these types of equipment:*

Diode CheckerGallium arsenide 1.2 VSilicon diode 0.7 VGermanium diode 0.3 VMany digital multimeters have a diode checking function. The diode should be tested out of circuit.A normal diode exhibits its forward voltage:*

OhmmeterAn ohmmeter set on a low Ohms scale can be used to test a diode. The diode should be tested out of circuit.*

Curve TracerA curve tracer displays the characteristic curve of a diode in the test circuit. This curve can be compared to the specifications of the diode from a data sheet.*

Curve Tracer*

Other Types of DiodesZener diodesLight-emitting diodesDiode arraysThere are several types of diodes besides the standard p-n junction diode. Some of common diode are:*

Zener DiodeA Zener diode is one that is designed to safely operate in its zener region; i.e., biased at the Zener voltage (VZ). Common zener diode voltage ratings are between 1.8 V and 200 V*More detail in next module

Light-Emitting Diode (LED)An LED emits light when it is forward biased, which can be in the infrared or visible spectrum. The forward bias voltage is usually in the range of 2 V to 3 V.*

*The Light-Emitting DiodeA large exposed surface area on one layer of the semi conductive material permits the photons to be emitted as visible light.The difference in energy between the electrons and the holes corresponds to the energy of visible lightWhen recombination takes place, at the pn junction the recombining electrons release energy in the form ofphotons

*The Light-Emitting Diode

*COLOREDLED

Typical LED CharacteristicsSemiconductor MaterialWavelengthColorVF @ 20mAGaAs850-940nmInfra-Red1.2vGaAsP630-660nmRed1.8vGaAsP605-620nmAmber2.0vGaAsP:N585-595nmYellow2.2vAlGaP550-570nmGreen3.5vSiC430-505nmBlue3.6vGaInN450nmWhite4.0v

Power Dissipation of a Diodeswhere ID and VD are the diode current and voltage at a particular point of operationPower dissipation in forward bias diode*

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